GH is about 6 coming out of the tap, according to API test kit. I don't get the white water line or crusty build up on my filter stem that is caused by high calcium load. It's kinda slimy once I start rubbing it off, but initially its just kind of a soft solid. I guess its biofilm, but its strange to me that it is no where else in my tank. Anywhere. Weird!
Might be something with the material that is on the airline tubing.
If you had a spare section of the material and place it on the opposite end of the tank, it should grow the same build up if that is the case.
Otherwise it might just have something to do with that area of the tank, if circulation isn't the best.
Don't know for sure though. Just guesses.